Microwave devices are generally divided into the broad categories of passive and active devices. Included under the heading of passive microwave devices are microwave hybrids which are multiport networks that are specifically configured for signal routing between the network ports. A device port into which power is normally fed is typically referred to as an incident port or an input port. A port from which power is extracted is called a coupled port or an output port and other ports (from which power is not extracted) are called isolated ports.
Microwave hybrids generally divide the power at each of a plurality of input ports into equal portions, transmit each of the divided portions to a respective one of a plurality of output ports and combine the transmitted powers at each output port. Accordingly, microwave hybrids are often called power divider/combiners.
An example of a four-port (2.times.2) power divider/combiner has two input ports and two output ports. In a perfect divider/combiner, the incident power at each input port would be divided into two equal portions which are each transmitted to a respective one of the output ports (i.e., the power division is perfect). None of the incident power would be reflected from the input ports and none of the power at any one of the input ports would be transmitted to the other ports (i.e., the input ports are perfectly matched to their power sources and the isolation between ports is perfect).
Most conventional power divider/combiners successfully divide powers received at input ports and combine these divided powers at output ports, they typically include an excessive number of transmission lines members and in some instances have degraded electrical performance, meaning high return loss, poor isolation and high coupling unbalance. Their use in microwave circuits, therefore, has a negative effect upon the size, weight and performance of these circuits. The weight of a microwave circuit is emphasized when the hybrid's transmission line members are realized in waveguide or coaxial form and the effect is especially costly when such realizations are intended for weight sensitive applications such as spacecraft.
An example of such a spacecraft application is an antenna array having a beam forming network which includes twenty-two coaxial 8.times.8 hybrids (each hybrid is formed with twelve 2.times.2 hybrids). Power divider/combiners that can be realized with less transmission-line members and smaller size would provide significant cost savings. Also, achieving flat coupling of the output powers will significantly enhance electrical performance and in particular coupling unbalance.